Penetrant inspection is a time proven, simple, and effective method ideal for locating surface cracks and flaws out in the field.

William E. Mooz, Met-L-Chek Co.

Figure 1. During inspection, penetrant is applied to the surface of the metal and allowed to enter flaws.

Figure 2. Remove excess penetrant from the metal surface. Some penetrant remains in any flaws.

Figure 3. Apply a white developer to the surface. The flaw shows up as a red indication on a white background

To detect leaks in vessel walls penetrant is applied to the inside wall and developer to the outside. In the event of a leak, penetrant bleeds through the wall, blots into the developer, and is revealed to the inspector.

The weld surface is given a final cleaning by moistening a clean cloth or towel with the remover, and giving the surface area a final wipe.

The developer sprayed on the weld as a fine,thin coating.

The developer will dry to a fine white coating, and any flaws will show up as a red indication on the white background.

Penetrant inspection was first developed by maintenance crews inspecting railroad rails who found that dirty engine oil placed on a rail read-ily entered surface cracks. They learned if they waited a bit, and then wiped the surface clean, oil remained in any cracks that might be present. Then, if they sprinkled whiting or chalk powder on the rail, the oil left in the cracks would be soaked into the chalk, and a dirty stain appeared on the chalk.

As technology developed, improvements included modifications to the oil carrier, and the addition of dyes to the oil so flaws were easier to see.

During World War II Loy W. Sock-man, working at Northrup Aviation, developed and patented formulations that located smaller flaws, and eventually founded Met-L-Chek Company.

Today, penetrant inspection materials are available for almost any application, ranging from the production line inspection of jet engine components to those used for field testing welds. However the basic principles remain the same.

Enter the Crack, Reveal the Flaw
The penetrant is a liquid that has two characteristics — the ability to enter fine cracks and some type of dye to make the cracks visible. This dye can be either an intense red or reddish-purple dye, or a dye that fluoresces when exposed to ultraviolet light. The type of dye depends on the type of inspection.

During an inspection, the penetrant is applied to the surface of the part to be inspected. If there are any surface flaws, the penetrant enters them, as illustrated in Figure 1.

The second step involves removing the excess surface penetrant from the part, leaving penetrant in any surface flaw illustrated in Figure 2.

The third step is to cause the penetrant in any flaws to rise to the surface, where it can be seen. To accomplish this, a developer is applied to the surface that blots the penetrant out of the flaw. The developer is white. If a red penetrant is used, the flaw shows up as a red indication on a white background, as shown in Figure 3.

Weld Inspection
Aerosol cans of the penetrant materials are typically used for weld inspections in the field. Kits contain penetrants, instruction manual, toweling, and a wire brush to clean the weld.

The first step is to clean the weld with a wire brush and penetrant remover. Penetrants cannot enter flaws filled with dirt or covered by paint or special coatings.

Once the weld is clean and dry, red penetrant is sprayed onto the surface and allowed to remain for approximately ten minutes to allow it to penetrate potential flaws.

Excess surface penetrant is then removed first by wiping the part with a clean dry towel cloth eliminating as much as possible, and then with a clean cloth or towel moistened with penetrant remover.

Next, the developer is applied by spraying a fine coating on the part, almost thin enough to be translucent. The developer will dry to a fine white coating, and any flaws will show up as a red indication on the white background.

Zeroing in on Fine Defects
Penetrant inspection is used in factories to locate extremely fine defects in high tech welds. A factory making vessels used in water purification needed to be certain that none of its vessels had leaks.

It needed to locate any flaws in their welds open to both sides. It chose an inspection technique that allowed it to locate fine defects.

In practice, the penetrant is applied to one side of the weld, such as the inside wall of the vessel. Then the developer is applied to the outside wall of the vessel. If there is a leak, the penetrant comes through the wall and is blotted into the developer, where it is seen by the inspector.

Fluorescent Lighting
Almost any penetrant will work for this application. However, when searching for extremely small leaks, it is easier to see penetrant indications that fluoresce under ultraviolet light.

In this application, the company switched from red penetrant to Met-L-Chek's FP-923, a high sensitivity water washable fluorescent penetrant allowing it to locate smaller leaks and clean up more quickly after inspection.

This method of inspection was easier on the inspector's eyes, because the indications really stands out in a darkened area.

Switching from the red penetrant method proved simple, but did involve performing the inspection in a darkened area with ultraviolet lights specifically sized for producing bright indications. However, quality improved, customer complaints declined, and profits increased.

William E. Mooz is president of Met-L-Chek Co. A Fellow of the American Society of Nondestructive Testing, and author of handbooks on the use of penetrant inspection materials. For more information:1639 Euclid St., Santa Monica, CA 90404; Phone: (310) 450-111; Fax: (310) 452- 4046. Edited by Ron Lucas, managing editor.